Well, there's one or two workers in TNA that WWE might want to take a look at, but like the NXT guys you mentioned they'll have new names, new-ish identitities and their wrestling past will only be referred to indirectly, unless you're one of the very top names outside of WWE like Samoa Joe, CM Punk, AJ Styles (and even then, there's Daniel Bryan). So basically what's already happening then.
There's no need for TNA guys to appear in the crowd, I'm repeating myself and others but TNA, as a brand, truly has zero worth for WWE - I'm sorry, I like some of TNA but it's true - ten years ago maybe we could have this discussion and mull over possibilities. Today it's pointless to even consider.
Of course, there's the fact Vince wouldn't want to treat TNA as a serious rival unless he really had to (which he won't have to, ever), but we can't forget the fact TNA have been in touch with SEVERAL promotions lately to do an invasion angle, eventually settling on GFW - who just need the exposure. Even companies like ROH and PWG want pretty much nothing to do with TNA at this point, it's a damaged brand.